Foam in paper mills and pulp mills may interfere with many processes and can affect the efficiency of refiners, pumps, screens, centrifugal cleaners, de-inking plants and water removal equipment. Foam will adversely affect final sheet formation, porosity, opacity, printability, smoothness, and plybond strength. Excessive foaming leads to loss of fibers and raw materials. Further, the drainage rate of a sheet on the wire can be affected by the presence of trapped air in a wet web. When a sheet or mat has a large number of trapped bubbles, the drainage rate is slowed because the water passing through the sheet or mat must also pass between the bubbles.
In many major industries and medicinal treatments, however, hydrophobic silica compositions are still the defoaming agents of choice. In the pulping and repulping of fibrous materials for the manufacture of waterlaid sheets such as paper, roofing felt, and the like, severe foaming problems can be encountered at several stages in the manufacturing process, particularly when natural organic materials having foaming properties are present when the fibrous mass is subject to strong agitation. Uncontrolled foaming can lead to decreased production, lower quality waterlaid sheet products, and other undesirable results. Foaming can also be encountered during bleaching, de-inking of broke or recycled pulp, cooking, washing, and draining operations, and the like. Various waste materials or waste or recyclable liquors can contain considerable amounts of foam and may require the addition of a defoamer.
Simethicone is a fluid antifoam or defoaming composition comprised of polydimethylsiloxane and silica suitably purified for its intended application. The preparation of liquid methylsiloxane polymers is delineated in U.S. Pat. No. 2,441,098, the disclosure of which is hereby incorporated by reference. The normal physical state of the simethicone Is a water white to grey translucent, viscous, oil-like liquid with a density of 0.965-0.970 grams/cubic centimeter having demonstrable immiscibility with water and alcohol.
The medically established therapeutic use for simethicone is as an ointment base ingredient, topical drug vehicle, skin protectant, but most particularly as an antigas and antiflatulent agent for human application as well as an antibloating agent for veterinary application.
Today's commercially available defoamers are capable of solving many industrial foam problems at surprisingly low concentrations, of as little as 0.0005% to about 3%, and preferably at concentrations of between 0.01-1%. Higher concentrations, although effective, become impractical. They are effective defoamers, particularly in aqueous systems wherein the foam is difficult to control, such as with industrial adhesive formulations.
Finely divided particles of silica have been used for foam inhibition in aqueous foaming systems. Typically, these silicas or silicates have been treated to make them hydrophobic, perhaps the most common treating agent being a silicone oil. The exact reasons for the effectiveness of the treated siliceous materials in aqueous foaming systems have not been fully or finally settled, but the scientific literature contains a considerable amount of theorizing on this subject.
The art has generally thought that both the hydrophobic fine silica and the silicone oil are necessary for foam inhibition in aqueous foaming systems. This conclusion is rationalized with the following theory: First, the hydrophobic silica is believed to be a foam breaker, i.e. the fine particles of hydrophobic silica help to rupture foam bubbles. Second, the silicone oil is essentially a carrier fluid which protects the filler particles and brings them into contact with the foam bubble surfaces.
The manufacture of hydrophobic silica and silicates has become an art in itself, and dozens of patents have issued regarding various aspects of this art. The structure of the substantially hydrophobic silica or silicate has not been determined with certainty. It is known from the literature that particulate siliceous materials having significant surface hydroxylation or surface silanol content can react chemically.
Two major methods are known in the art to produce the hydrophobic formulation. These methods are the "Dry Roast" method and the "In-Situ" method. In both methods, the silica is treated with silicone oil in amounts of about 5 to 40 wt. %. In the "Dry Roast" method, the silica as a dry powder is treated or impregnated with the silicone oil by heating, for example, at about 205.degree. C. for about 4-6 hours to coat the silica with the silicone fluid. The resulting coated silica is then dispersed in a carrier fluid such as mineral oil, edible oil, silicone oil, fatty acid, fatty esters, polyalkoxylate esters, water, or mixtures thereof to form the defoaming agent.
In the second method, the "In-Situ" method, the silica dry product, the carrier and silicone fluid are mixed and heated at about 105.degree.-140.degree. C. for 4-6 hrs in the presence of a catalyst (e.g., an amine, diamine or ammonium carbonate) to form a homogenous mixture and convert the silica to hydrophobic particles and form the defoaming formulation in a single step.
Unfortunately, both methods have certain drawbacks. The dry roasting method requires large, expensive apparatus for processing commercially justifiable volumes of low density silica powders. In many instances, the cost of larger equipment effectively limits the throughput of the overall facility.
The conventional in-situ method offers the advantage of a liquid medium that counteracts the low density/high volume process limitations of the dry roast process at the cost of introducing catalyst/promoter residues into the composition. The adverse effects of such residues must, in turn, be counteracted by adding still other ingredients (such as an adjustment material) that may adversely limit the applicability of the resulting composition.
It would be desirable to have a process for making hydrophobic silica particles with a semi-batch or continuous step.
It would also be desirable to have a manufacturing process that was free of catalyst/promoter component residues in the hydrophobic particle dispersion product.